BOSTON -- When Jon Lester takes the mound today at Fenway Park, he’ll do so in the midst of a pretty good stretch by modern major-league standards. During four June outings he’s pitched into the seventh inning of each, and not allowed more than three runs in any, which gives him a quartet of quality starts. By definition, that’s certainly decent.

But decent isn’t good enough for Lester.

He is the opening day starter. He is the presumed ace. He is supposed to be in his prime. Yet the 28-year-old lefty lugs a 4.48 earned run average into today’s matchup with the Blue Jays, ranking 31st among 48 American League qualifiers and half a run worse than the typical big-leaguer. Some have said he’s elite, but this year he hasn’t even been average.

And given the current state of the team’s starting pitching, the Red Sox need that to change. Now.

His 3.71 ERA this month suggests he’s trending in that direction, as does the fact he appears to be past some early season command issues. There are numbers that suggest some of his struggles have been the result of bad luck, considering opponents are batting .325 against him on balls in play, and his fielding independent pitching number — which attempts to remove defense from the evaluation of a pitcher — actually ranks 15th in the AL at 3.25.

But based on the unexpected direction Lester’s career has been heading over the past season and a half, he needs to be better than that, and sustain it, before anybody should believe that Lester is really the stopper he once seemed certain to be.

At 24, in his first full season, Lester went 16-6 with a 3.21 ERA and headed the rotation in the postseason. The next year he struck out 10 batters per nine innings. Then a year after that he went 19-9, with a 3.25 ERA, a league-best 9.7 whiffs per nine, a career-best 1.20 walks and hits per inning pitched, and fourth place in AL Cy Young voting.

At 26, there was three years’ worth of evidence to suggest Lester belonged in the conversation with C.C. Sabathia, Cliff Lee and Cole Hamels as the best southpaws in the game — though since then that’s changed. He was OK for most of 2011, but he struck out 43 fewer hitters than he did a season prior, coughed up 20 homers, threw only 191 2/3 innings and was brutal in September.

That carried over into this year when, though his first two starts were good, Lester posted a 5.47 ERA as opponents tagged him with an .824 on-base plus slugging over his next nine starts. He’s recovered some, but still his strikeouts are down to just 7.2 per nine, he’s allowing more than a hit per inning for the first time since he was a rookie, and in two of his last four appearances he’s let a Boston lead slip away in the seventh inning.

That’s a failure as an ace. So is seeing the team lose six of the nine starts he’s made following a loss. And the Sox can’t afford those failures when that’s exactly what they need Lester to be.

Josh Beckett may return to the rotation in Seattle this weekend, but for the time being only Lester and Felix Doubront remain from the Sox’s original starting staff — and Doubront has lately looked homer-prone and vulnerable. With them now are the inconsistent Daisuke Matsuzaka, the unspectacular Aaron Cook and converted reliever Franklin Morales.

And with Beckett no sure thing after already experiencing back and shoulder problems, Clay Buchholz still hospitalized with gastrointestinal bleeding caused by esophagitis, and Daniel Bard going back to the bullpen in Pawtucket, that cast isn’t likely to soon change much — making it all the more important that Lester take his game to another level.

If the Red Sox are going anywhere, he needs to build on the positives of his past four starts, because the team isn’t built to win if he can’t. With so many other question marks, the Sox need to know they have a certainty every fifth day — and that is supposed to be Lester. In his seventh season, he’s been entrusted that responsibility, the expectation, that burden.

Now he’s got to prove that he deserves it.

- - - - - - -

ACCORDING to the Red Sox, Buchholz “has esophagitis, which led to an erosion of the esophagus and an associated gastrointestinal bleed. He has been evaluated and observed at Mass General Hospital. Clay is doing well and is expected to make a full recovery.”

Bobby Valentine added no insight as to when he may get back to the mound, but the manager did acknowledge the possibility that the Sox could temporarily go to a six-man rotation when Beckett is activated this weekend. That would leave the team in good shape for a July 7 doubleheader against the Yankees, and also build in some additional rest around the all-star break.

- - - - - - -

JACOBY ELLSBURY could begin a rehab stint in the Gulf Coast League as early as Friday. Carl Crawford has been there a few days already, though rainouts have spoiled both of his planned opportunities to play left field.

- - - - - - -

THE New Hampshire Fisher Cats may be the Double-A team of the Blue Jays, but their alumni haven’t made much of an impact in this series. Monday night, the battery of Henderson Alvarez and J.P. Arencibia were the only Toronto starters who’d spent more than a rehab assignment in Manchester, then Tuesday first baseman Adam Lind was the lone ex-Fisher in the lineup.

- - - - - - -

WHEN he was traded, many suggested Kevin Youkilis had nothing left. But consider this: Through June 25 of the season each played at 33 years old, Youkilis was hitting .233, with a .314 OBP, .687 OPS and 4 homers with a 23.1 percent strikeout rate — while David Ortiz was at .216, .309, .690, 7 and 22.9 percent.

Three years later, Ortiz entered Tuesday with a .304 average to go with 20 taters and 52 RBIs.

Dave D’Onofrio covers the Red Sox for the New Hampshire Union Leader and Sunday News. His e-mail address is ddonof13@gmail.com.